Devices having multiple power supply inputs may balance current drawn from these inputs to draw maximum power from all supplies without overloading individual inputs. Typically, the current balancing is performed using either resistive ballasting or active balancing.
Resistive ballasting involves adding series resistance to each input. As the current draw on one input increases, the voltage drop across the resistance on that input increases proportionally, decreasing the voltage available to the device from that input and causing it to draw more current from the other inputs. However, resistive ballasting tends to waste power in the resistors, especially when two input voltages are significantly different from each other.
Active balancing typically involves inserting a resistor in series with each input and measuring the current in each input across this resistor. The resulting signals are used to actively adjust individual regulators attached to each input to increase or decrease their percentage of the total current draw. This technique typically improves efficiency over the resistor ballasting scheme, but it is complicated and typically involves a large amount of specialized circuitry.
Therefore, it would be desirable to develop simple and efficient techniques for balancing a current draw from multiple power supply inputs.